Thin film electroluminescent (EL) displays are well known in the art and are used as flat screen displays in a variety of applications. A typical display includes a plurality of picture elements (pixels) arranged in rows and columns. Each pixel comprises an EL phosphor active layer between a pair of insulators and a pair of electrodes.
Early EL displays were only operated in a multiplexed mode. Recently active matrix technology known in the liquid crystal display art has been applied to EL displays. A known AMELD includes a circuit at each pixel comprising a first transistor having its gate connected to a select line, its source connected to a data line and its drain connected to the gate of a second transistor and through a first capacitor 22 to ground. The drain of the second transistor is connected to ground potential, its source is connected through a second capacitor to ground and to one electrode of an EL cell. The second electrode of the EL cell is connected to a high voltage alternating current source for excitation of the phosphor.
This AMELD operates as follows. During a first portion of a frame time (LOAD) all the data lines are sequentially turned ON. During a particular data line ON, the select lines are strobed. On those select lines having a select line voltage, transistor 14 turns on allowing charge from data line 18 to accumulate on the gate of transistor 20 and on capacitor 22, thereby turning transistor 20 on. At the completion of the LOAD cycle the second transistors of all activated pixels are on. During the second portion of the frame time (ILLUMINATE), the AC high voltage source 28 is turned on. Current flows from the source 28 through the EL cells 26 and the transistor 20 to ground in each activated pixels, producing an electroluminescent light output from the activated EL cell.
This AMELD and known variants require a number of components at each pixel and do not have gray scale operation. Thus there is a need for alternative AMELDs having fewer components and gray scale operation.